I use a Mac simply because I feel OS X is the better operating system. Linux has made strides on the desktop but when push comes to shove, it's just not "there" yet. I'm sure I'm going to be downmodded for saying that, but I really feel it's the truth. Desktop Linux is unstable. While it doesn't crash often, it does sometimes, and I feel that one crash is too many. Multiple displays are wonky. Flash still doesn't work properly. Etc. Etc.
I use OS X because it has the UNIX underpinnings I need to do my job with the stable desktop and cohesive UI that I feel Linux is missing.
Unlike many Linux zealots, I don't feel that there is anything "wrong" with closed-source software. I didn't use Linux for years because it was free as in beer or free as in freedom. I used it because I thought it was the better tool.
And that's why I use a Mac.
Here is my biggest concern though: Apple is an evil, proprietary company. In my opinion, worse than Microsoft. If you use OS X, you are legally locked in to Apple hardware. You cannot legally run OS X on anything but Apple-bought-and-branded computers. At least, with Windows, I have the freedom to install the software on any bloody hardware I want. Yeah yeah, you can Hackintosh or whatever, but it goes against Apple's EULA.
Just because Apple releases parts of OS X under open source licenses, doesn't mean OS X isn't completely proprietary. You can only run it on what Apple lets you run it on. If you need to upgrade your workstation(s) and want to stick with OS X, you are completely at mercy to Apple's products and prices.
In the end, I suck it up. Hopefully one day this will change, but right now OS X is a pleasure to use, and that outweighs the dislike I have with the license.